1. Field of the Invention
This invention relates to lamps and more particularly to lamps using solid state devices as a light source.
2. Description of the Related Art
Conventional tungsten based lamps and indicator lights convert electrical current to light by applying a current to a filament, which causes it to glow. The filament is commonly suspended near the center of a glass bulb between two rigid current leads, which allows for a radial distribution of the light that is particularly useful for room illumination. The surface of the bulb can also be frosted to cause additional scattering of the light. The life-span of filament based lights is relatively short and is usually limited by the life-span of the filament or of the glass bulb. In addition, the filament is usually suspended close enough to the bulb surface that heat from the filament can cause the bulb to become very hot, such that it is painful to the touch or presents a danger of burning objects that come in contact with it.
Light emitting diodes (LEDs) are an important class of solid state devices that also convert electric energy to light. They generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted omnidirectionally from the active layer and from all surfaces of the LED.
Most conventional LEDs are less efficient at converting current to light than filament lights, but recent advances in nitride based LEDs have resulted in highly efficient blue and green light sources. The efficiency of these LEDs has already surpassed that of filament based light sources, providing a light with equal or greater brightness in relation to its input power.
One disadvantage of conventional LEDs used for lighting applications is that they cannot generate white light from their active layers. One way to produce white light from conventional LEDs is to combine different colors from different LEDs. For example, white light can be produced by combining the light from red, green and blue LEDs or blue and yellow LEDs. One disadvantage of this approach is that it requires the use of multiple LEDs to produce a single color of light, increasing the overall cost and complexity. In addition, different colors of light are often generated from different types of LEDs and combining different LED types on one device can require complex fabrication. The resulting devices can also require complicated control electronics, since the different diode types can require different control voltages. The long term wavelength and stability is also affected by the different aging behaviors of the different LEDs and the miniaturization of the multi-LEDs is limited.
More recently, the light from a single blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Hayden, xe2x80x9cMultiple Encapsulation of Phosphor-LED Devicesxe2x80x9d]. The surrounding material xe2x80x9cdownconvertsxe2x80x9d the wavelength of at least some of the LED light, changing its color. For example, if a nitride based blue emitting LED is surrounded by a yellow phosphor, some of the blue light will pass through the phosphor without being changed while the remaining light will be downconverted to yellow. The LED will emit both blue and yellow light, which combines to provide a white light.
However, conventional blue LEDs are too dim for many lighting applications currently using filament based lamps or indicators, and when used to produce white light, some of the emitted light can be absorbed by the downconverting material. For blue LEDs to emit an output light flux sufficient for room illumination, the current applied to the LED must be increased. LEDs commonly operate from a current of 20-60 mAmps, which must be increased to greater than 1 Amp for the LED to illuminate a room. At this current level, LEDs become very hot and can cause damage to other objects and present a danger of fire or injury. The heat can also damage the LED chip itself, or degrade nearby downconverting media such as phosphors, fluorescent polymers, or fluorescent dyes. This can reduce the LED""s downconverting ability and decrease its useful lifetime.
Another disadvantage of most conventional LEDs is that they are seated in a xe2x80x9cmetal cupxe2x80x9d, with the n-type layer typically at the bottom of the cup and the p-type surface directed up, with the entire device encased in a clear epoxy. When a blue emitting LED is used for generating white light, it is surrounded by a downconverting material before being encased in the clear epoxy. The cup has two conductive paths to apply a bias across the contacts on the p and n-type surfaces, which causes the LED to emit light. The cup also reflects light emitted from the LED""s bottom n-type or side surfaces, back in the direction of the p-type upper surface where it adds to the light emitted from the LED. However, this reflection also causes the light source to be highly directional, so that it is brightest when viewed from directly above the LEDs emitting surface. Furthermore, this conventional LED emits light that appears a different color when viewed from different angles. This is due to incomplete mixing and randomization of the light rays of different colors. This type of light is not useful for room illumination or other applications requiring a dispersed light source or uniform color illumination.
To overcome this limitation, various LED lamps have been developed which use multiple directional LEDs arranged to provide a radial type light source. [See U.S. Pat. No. 5,688,042 to Abolfazl et al., U.S. Pat. No. 5,561,346 to Byrne, U.S. Pat. No. 5,850,126 to Kanbar, and U.S. Pat. No. 4,727,289 to Uchida]. However, because these lamps rely on multiple LEDs, their cost and complexity is increased. Also, they can only provide one pattern for dispersing the light. U.S. Pat. No. 5,931,570 to Yamura discloses a light emitting diode lamp in which the LED is embedded in one end of an epoxy bulb shaped body with the LED leads extending from the bulb end. The body also has a convex top that is frosted to disperse light from the LED. One disadvantage of this LED lamp is that it is not capable of producing white light without being combined with additional LEDs of different colors. In addition, if current were supplied to the LED for room illumination, the LED would become dangerously hot and could be damaged. Also, the lamp can only diffuse the light in one pattern that is hemispheric at best. Very little light is visible if the lamp is viewed from the its back side.
Solid state semiconductor lasers convert electrical energy to light in much the same way as LEDs. They are structurally similar to LEDs but include mirrors on two opposing surfaces, one of which is partially transmissive. In the case or edge emitting lasers, the mirrors are on the side surfaces; the mirrors provide optical feedback so that stimulated emission can occur. This stimulated emission provides a highly collimated/coherent light source. A vertical cavity laser works much the same as an edge emitting laser but the mirrors are on the top and the bottom. It provides a similar collimated output from the its top surface.
Solid state lasers can be more efficient than LEDs at converting electrical current to light, but their coherent light output is not useful for lamps because it only illuminates a small area. Also, they cannot efficiently produce green or blue light and their relatively small beam areas makes it impractical to combine the output of multiple different colored lasers.
The present invention provides a new solid state lamp that can disperse light in many patterns, but is particularly applicable to a radial dispersion of white light that is useful for room illumination. The new lamp consists of a separator/light transport medium (xe2x80x9cSeparatorxe2x80x9d) with a light source at one end and a light dispersing/frequency converting element (xe2x80x9cDisperserxe2x80x9d) at the other end. The new lamp can also have an enclosure to protect the three primary components and/or to provide additional dispersion or conversion capabilities.
The light source consists of at least one solid state light emitting device such as solid state LEDs or solid state semiconductor lasers, with some or all of the light directed down the Separator. When using more than one LED or laser, the devices can emit light with either similar or different wavelengths, depending on the intended application. For example, blue and yellow emitting LEDs can be used together to produce white light. The light source can incorporate additional components to aid in thermal management and electronic power management and can also include light reflecting or focusing elements. It can be contained within an enclosure such as the traditional screw socket end of a tungsten bulb to provide for heat and/or power transfer to the outside environment.
The Separator provides physical separation between the light source and the Disperser and also guides light from the light source to the Disperser. If desired, the Separator can actively shape, collimate, disperse, and/or actively guide the light from the light source to the Disperser. The Separator can contain materials that convert the wavelength of some or all of the light, or scatter or focus the light into specific patterns. It can also incorporate solids, liquids, or gases, to further interact with the light.
The Disperser is spaced from the light source by the Separator and serves primarily to disperse the light from the light source. It may also modify the wavelength of light from the source by including one or more converting materials which absorb some or all of the incident light and re-emit light of different wavelength. In one embodiment, the Disperser is the hemispheric shaped end of the Separator with a roughened surface for light scattering. It can also be removable to allow for the use of different Dispersers and can include a variety of optical components to shape the light such as lenses, reflectors, granules, holographic elements, or microspheres. A mask or reflector may be used to provide a more directional or controlled light distribution, or patterned illumination such as a company logo or sign.
The separation between the light source and the Disperser gives the new lamp a number of advantages and allows for flexibility in converting light from the light source into useful room illumination. The Disperser can take many different shapes to provide different light dispersions patterns, such as a uniform radial distribution. When operating at elevated current levels, the majority of the heat generated by the light source will be dissipated prior to reaching the Disperser. This prevents the Disperser from becoming dangerously hot and damaging any converting materials it may contain. The separation permits bulky thermal elements, such as heat sinks or heat fins as well as electronic components, to be placed near, or attached directly to, the Light Source without interfering with the light distribution. Accordingly, the separation allows a solid state light source, operating at elevated current levels, to safely and efficiently provide light useful for room illumination.
Compared to filament based lamps, the new lamp is more robust, lasts longer and in the event of failure or damage portions of the lamp can be replaced, without having to replace the entire lamp. For example, a Disperser containing downconverting materials such as fluorescent dyes, phosphors, or polymers, will generally have a shorter lifetime than the light source, and the ability to replace the Disperser by itself provides a significant cost savings.